It’s pretty simple. Side-loading can be exploited by social engineering.

Two very bad consequences for me if sideloading were easy:

1. There would be no safe product to recommend to my aging parents who would be vulnerable to social engineering.

2. A company with a popular produce like Facebook could go outside the store, which would normalize side-loading, thus rendering argument like ‘people who want safety can stick to the store’ moot.

The arguments about not wanting to control what other people do are moot. If you want a platform with side-loading, buy android.